The present invention relates to an apparatus for the extrusion of a molten polymer material into a plurality of yarns, whereby this apparatus is provided with distribution channels through which the molten mass is fed to spinning nozzles, as well as with heating devices in order to keep the material flow at the required temperature for extrusion until emergence from the nozzle bores, and furthermore with a cooling system with a blower nozzle associated with the nozzle bores having a slit-shaped outlet opening which is directed upon the yarns emerging form the nozzle bores so that the latter are subjected to the air flow coming out of the blower nozzle.
Filaments of yarn-forming polymers such as polyester, polyamide and polyolefine are normally produced in the melt-spinning process. In this process, the polymer is melted down and homogenized in an extruder and is then fed through a melt line to spinning pumps which press the molten mass through nozzle bores. The filaments emerging from the spinning nozzles are continuously cooled by an air stream. The filaments thus produced are then prepared, gathered together into a cable, drafted, crimped and cut into staple fibers.
The first steps of the process have a great influence on the subsequent process steps with regard to product quality, so that even before or during the yarn formation the final quality of the yarns or staple fibers is decidedly determined.
The process starts with the extrusion, which must deliver as homogenous a molten mass as possible with respect to temperature distribution, and if applicable, also distribution of additives.
The distribution of the molten mass among the nozzle bores takes place therefore in precisely stepped channel system with as identical channel lengths as possible for all spinning pumps. Constant temperature is ensured by means of heating of all channel segments in as uniform a manner as possible. Gear pumps with frequency-regulated drives provide precise dosage of the individual volume flows to the nozzle bores.
In practice, different shapes of the spinning nozzle packages, i.e. rectangular, annular and round-nozzle packages are used, depending on the application, and for these as uniform a distribution of the molten mass must ben ensured in turn over the entire output surface of the molten mass.
A distribution system of this type for a ring-nozzle package is shown for example in EP 0 517 994, where the molten mass is conveyed through a complicated system of channels of equal length to the nozzle bores of a ring-shaped nozzle package. In U.S. Pat. No. 4,259,048 an annular nozzle package is described in which the distribution of the molten mass is ensured by a plate-shaped space.
However, these solutions function faultlessly only if all the channels are at the same temperature, as otherwise differences in the flow capability of the polymer appear due to the temperature difference, and the molten mass flows preferably through the warmer channels according to the principle of least resistance. This expresses itself clearly in the difference in titer and is expressed in the end product through the variation coefficient. Furthermore, such temperature irregularities have a detrimental influence upon draftability, so that high product quality cannot readily be achieved. This applies in particular to the production of very think fibers or filaments, since the inherent enthalpy is lower, due to the lower throughput, and therefore the influence of temperature on the viscosity of the molten mass is greater because of the longer dwell time.
In practice it has been found however, that even in a housing heated with a vaporous heat-carrying oil with seats for the spinning nozzles, the so-called spinning beam, temperature differences occur because the nozzles cannot be installed without gaps. Gaps serve to compensate for deformation processes when seals are crushed, or are required as safety clearances because of the heat expansion occurring at the process temperatures between 220 and 300.degree. C. Because of these gaps, the optimal heat transfer through heat conduction can be used only to a very limited extent.
The utilization of electrical heating elements (DE 4 312 309 C2) which can be screwed or clamped directly on the spinning nozzle package, offers a solution. This means however, that the energy required for heating must be supplied through wires and contacts which must be removed when a spinning nozzle package or even parts of same must be disassembled, and this considerably increases the cost and also leads to increased wear of the parts involved. Furthermore, energy losses occur in the wires and contact bridges. Another aspect here is the danger to machine operators due to electrical voltage which appears on these heating elements and may endanger personnel in case of improper handling.
The direct thermal oil heating system also represents a considerable complication in installation and removal of the spinning nozzles or of the nozzle package, since oil pollution can be expected during the establishment or the removal of the connections. Furthermore these residues interfere with the cleaning of nozzle bores in cleaning devices. Local temperature adaptation in case of interfering influences is not possible.
The uniformity of yarn cooling at emergence from the spinning nozzle package has as much importance as even distribution of the molten mass. Here it must be ensured that each filament is cooled with the most uniform air temperature possible on the same path and at the most uniform speed possible. Cooling at the congealing point of the molten mass is of the greatest importance here, since the momentary position of the still freely moving and already pre-oriented molecules of the synthetic material is frozen. Differences in cooling result as a rule in differences in draftability and, in the worst case, to yarn breakage already in spinning. To reduce these influences, lateral blowing, inflow cooling (from the outside to the inside) and outflow cooling (from the inside to the outside) is applied, depending on the nozzle shape used. Depending on the density of the field of perforation, on the width of the field of perforation, on polymer and throughput, the air velocity varies between 0.1 and over 20 m/s.
All cooling processes and air velocities have the common problem that the blown air heats up as it penetrates the filament group from filament to filament and has therefore a distinctly higher temperature at its output than on the input side. Consequently, the filaments on the output side receive different cooling from the filaments on the input side of the cooling air. As described earlier, this leads to differences in draftability and to reduced product quality. In addition, the movement of the filaments and the resulting drag flow cause a certain amount of deflection of a previously well directed air stream, so that the uniformity of the congealing point between input and output side is also no longer ensured.
EP 0 536 497 therefore describes a cooling system with outflow blowers which consist of two units with different orientation and with substantially independent adjustability with respect to blown air flow to reduce the temperature differences or to compensate for the blown air deflection. This blown air stream is expensive and yet not satisfactory.